The present invention generally relates to a method and apparatus for detecting errors in the transmission of multi-bit data words along a data channel, such as during the recording and reproducing of the data words by a magnetic recording and reproducing apparatus.
The error detecting apparatus of the present invention, while being applicable to many kinds of data transmission, has particular usefulness in magnetic recording and reproducing and it is in such an environment that the present invention is described.
The video tape according and reproducing apparatus that is currently most widely used in commerical quality television broadcasting is known as quadruplex format recording apparatus, which has recorded tracks oriented substantially transverse to the longitudinal direction of the tape. This is accomplished by a rotating wheel typically carrying four equally circumferentially spaced transducing heads which record the video information on the tape as well as reproduce the same during playback or reproducing. Such commercial grade recording apparatus utilize record and reproduce FM signals which have attendant undesirable attributes that continue to exist, in spite of considerable attention that has been focused on them by extremely skilled scientists and engineers. Degradation of the resulting video signal after recording and reproducing is one of the more significant undesirable attributes and it can be due to many causes. Degradation is experienced in the form of Moire, head banding of various types due to mechanical tolerances being exceeded, noise, transients caused by switching of heads and time base errors resulting from changes of tape dimension due to humidity, temperature and servo induced instability and the like. The FM signal is also quite vulnerable to medium surface irregularities, such as scratches, which may be present on the magnetic tape and which affect the signal that is obtained during reproducing. Such recorders are also sensitive to so-called cycle hops and experience degradation during multiple generations of a recording as might occur during an editing process or during reproducing additional copies of a video tape recording. While the reproduced video signal can be applied to a digital time base corrector for the purpose of correcting time base errors, the signal that is obtained off tape which is to be time base corrected nonetheless contains noise, Moire, head switching transients and tape dimension and servo induced errors, all of which can effect the sampling of the analog FM signal for purpose of time base compensation and therefore result in undesirable changes in the subcarrier phase which affect the resulting color and signal timing that is subsequently obtained.
The present apparatus does not utilize FM recording, but records and reproduces a digital video signal that is pulse code modulated using an encoding technique that will be hereinafter described. The digital recording and reproducing apparatus offers many significantly improved operational characteristics compared to FM recording and reproducing as will be described herein. Among the desirable attributes of the apparatus described herein is the virtual elimination of banding and Moire of any nature from any cause, the reduction of chroma and luminance noise to a value better than -54 db, the ability to relax mechanical tolerances for quadrature adjustments by a factor of about 100, the presence of an inherent time base error that is no greter than about 1/2 nanosecond. Additionally, the apparatus enables perfect color framing to be obtained at all times and introduces virtually no degradation druing regeneration of the reproduced signal, which means that essentially unlimited numbers of generations of video tape recording can be made. Moreover, since the decision for making the zero crossing selection for digitally sampling the analog video signal is precisely determined with respect to the location of the horizontal synchronization pulse before recording, the apparatus is completely immune to cycle hops, which are a problem with present video tape recorders having to make a decision at each start of reproducing. Also, tape irregularities, such as scratches and surface roughness which cause drop-outs in FM recording is significantly less consequential to the operation of the system described herein, which means that less expensive video tape can be used to produce a significantly improved quality recording compared to an FM recording.